You are asking about the speed restriction on the FMC climb and descend pages, right? If so:
240/100 is with reference to either 10.000' MSL or FL100 STD.
Which one, is decided by the:
1) Airport/departure (for climb) you have loaded in the FMC.
2) Airport/approach (descent) you have loaded in the FMC.
The TA/TL is saved in the database together with the above mentioned procedures. If you do not select a procedure (i.e. an airport with out a departure procedure), the FMC assumes TA is 18.000 feet (can be set manually though on take-off reference page 2 manually). Hence the speed restriction will be 10.000' MSL.
Under no circumstances does the 240/100 refers to AGL. That means that if your airport is at 7.000', the VNAV will keep your descent speed until reaching 10.000' MSL or 3.000' AGL before reducing to 240. Unless you manually change the entry to e.g. 240/170, should you so desire.
Hope I wrote it in a way that doesn't cause you further confusion.